The present invention relates to a travel assisting hydraulic circuit for a hydraulic drive type working vehicle, and the invention particularly relates to a travel assisting hydraulic circuit for a hydraulic drive type working vehicle equipped with a working machine, which travels by driving drive wheels by means of a hydraulic pump and a hydraulic motor driven by an engine.
Conventionally, a hydraulic drive type working vehicle (hereinafter simply referred to as a working vehicle), in which a traveling hydraulic pump (hereinafter referred to as a traveling pump) and a hydraulic pump (hereinafter referred to as an assisting pump) which is used for a purpose other than traveling (for example, driving a working machine) are driven by an engine is known. Such a vehicle travels by means of a hydraulic static transmission circuit (hereinafter called an HST circuit) which drives drive wheels by rotating a traveling motor with discharge pressurized oil of the traveling pump. Also, discharge pressurized oil of the assisting pump operates a working machine by extending and contracting a working cylinder, for example.
As an assisting hydraulic circuit for such a working vehicle, for example, a hydraulic circuit disclosed in Japanese Patent Laid-open No. 9-32045 is cited. According to the disclosure, in a hydraulic drive type working vehicle equipped with a traveling HST circuit and a working machine hydraulic circuit, a flow combining and dividing valve, which combines discharge oils from a traveling pump and a working machine hydraulic pump into one circuit, or divides them into each of their own circuits. By means of the flow combining and dividing valve, when the pressure of the traveling HST circuit is lower than a first predetermined pressure and the engine speed is higher than a predetermined value, the discharge oil from the working machine hydraulic circuit combines into the traveling HST circuit. On the other hand, when the pressure of the traveling HST circuit is higher than the first predetermined pressure, the discharge oil from the working machine hydraulic circuit is stopped from combining.
As a result, when the engine rotates at a higher speed and traveling load is lower, the discharge pressurized oil from the working machine hydraulic pump assists the traveling hydraulic motor, therefore rotating the traveling hydraulic motor at a speed higher than the rotating speed corresponding to the maximum discharge quantity of the traveling pump, and allowing the working vehicle to travel at a high speed.
However, the hydraulic circuit of the hydraulic drive type working vehicle disclosed in Japanese Patent Laid-open No. 9-32045 has the following disadvantages.
(1) The pressure of the traveling HST circuit and the pressure of the discharge oil added by the working machine hydraulic pump are the same, therefore requiring means for protecting the circuit of lower rated pressure. As a result, the configuration of the circuit is complicated, or effective assistance cannot be obtained since assistance can be provided only in the limited pressure range.
(2) Conventionally, a closed type (specifically, a circuit composed of a traveling pump and a traveling motor is basically a closed circuit), and an open type (specifically, the traveling pump and the traveling motor are connected via a change-over valve or the like, and the circuit composed of them is an open circuit) are used as a traveling HST circuit. Accordingly, it has been strongly desired that the travel assisting hydraulic circuit is made applicable to the aforesaid closed type traveling HST circuit. The hydraulic circuit disclosed in Japanese Patent Laid-open No. 9-32045, however, has the configuration only applicable to the open type.
(3) Normally, a traveling pump and a traveling motor of a large capacity are required in order to travel at a high speed. The traveling pump and the traveling motor of a large capacity, however, are larger in size, therefore making it difficult to decrease the entire size of the traveling HST circuit. In addition, a hydraulic pump and a hydraulic motor of capacity large enough to be capable of traveling at a high speed are less available, and further it is difficult to produce them, therefore causing the disadvantages of performance and cost, which makes them impractical.
Mitigating the aforesaid conventional disadvantages, an object of the present invention is to provide a traveling assisting hydraulic circuit for a hydraulic drive type working vehicle with more general versatility, in which a travel assisting circuit during travel at a high speed can be composed of a simple circuit with use of a hydraulic pump and a hydraulic motor of small size and capacity irrespective of the type of a traveling HST circuit (a closed circuit or an open circuit, or the number of pumps or motors).
In order to attain the aforesaid object, a travel assisting hydraulic circuit for a hydraulic drive type working vehicle according to the present invention is, in a hydraulic drive type working vehicle which includes a traveling HST circuit having a traveling pump driven by power of an engine and a traveling motor driven by pressurized oil from the traveling pump, with the output torque of the traveling motor being transmitted to a drive wheel via a speed reducer, characterized by including an assisting pump driven by the power of the engine, and an assisting motor for outputting torque generated by being driven by the pressurized oil from the assisting pump to the speed reducer to add the torque to the output torque of the traveling motor.
According to the above configuration, the traveling assisting hydraulic circuit, namely, the circuit of the assisting pump and the assisting motor, is constructed independently from the traveling HST circuit, therefore making the travel assisting hydraulic circuit applicable irrespective of the type of the traveling HST circuit (the numbers of hydraulic pumps and hydraulic motors, a type of circuit, and the like). Accordingly, the travel assisting hydraulic circuit is increased in general versatility, and commonality of the devices used is made possible. The rated pressure of the travel assisting hydraulic circuit is not influenced by the pressure value of the traveling HST circuit, therefore simplifying the configuration of the travel assisting hydraulic circuit, and enabling to assist in providing torque in the entire higher speed range of the working vehicle. Further, the hydraulic pump and the hydraulic motor of a small size and capacity can be used in the traveling HST circuit and the travel assisting hydraulic circuit, thus easily decreasing the size of the entire apparatus and achieving commonality of the devices used.
Further, the traveling HST circuit may be an open circuit or a closed circuit. According to the configuration, the travel assisting hydraulic circuit is applied to all hydraulic drive type working vehicles in which the traveling HST circuit is composed of an open circuit or a closed circuit, therefore enabling to construct the travel assisting hydraulic circuit increased in general versatility.
Furthermore, during inertia traveling, or while a brake is operated, the assisting motor may be used as an energy regenerating pump. Conventionally in general, while the vehicle travels by inertia, or while a brake device is operated, kinetic energy during traveling, which the vehicle itself has, is consumed by producing heat for braking. On the other hand, according to the present configuration, while the vehicle travels by inertia on a downward slope, or while the brake is operated, the assisting motor works as a regenerative pump. The discharge pressurized oil during regeneration is stored in an accumulator with the assisting motor, thereby making it possible to use the stored energy as a hydraulic source for the other devices, and therefore an energy-saving working vehicle can be constructed.
Further, the assisting pump and the assisting motor are either a fixed displacement type or a variable displacement type. According to the configuration, types of the assisting pump and the assisting motor have no limitation, therefore making it possible to construct a travel assisting hydraulic circuit increased in general versatility, Consequently, when the assisting pump is a variable displacement type, a fixed displacement type of assisting motor is used, and when the assisting pump is a fixed displacement type, a variable displacement type of assisting motor is used, thereby simplifying the configuration of the travel assisting hydraulic circuit and facilitating the control. In this way, the assisting motor can be selected according to the type of the assisting pump. Furthermore, the types of the assisting pump and the assisting motor can be selected according to the purpose of the assisting motor, therefore eliminating restriction in selecting the devices.
The travel assisting hydraulic circuit for the hydraulic drive type working vehicle characterized by further including steering cylinder for driving steering, and a steering pump for supplying pressurized oil to the steering cylinder, with the assisting motor being used as an alternative to the steering pump either when the engine stops, or when the steering pump is out of order.
Conventionally, in general, when the engine or the steering pump is out of order, pressurized oil is stopped from being supplied to the steering cylinder, thereby making the steering operation impossible. As a result, the operability is greatly decreased when the working vehicle is towed to move. On the other hand, according to the present configuration, the assisting motor works as an alternative to the steering pump under emergency conditions by towing or pushing the working vehicle. Accordingly, even when the engine stops, or the steering pump is out of order, the steering operation of the working vehicle becomes possible, thereby increasing steering operability under emergency conditions. In addition, it is not necessary to specially provide a steering pump used under emergency conditions (a so-called emergency steering pump), therefore simplifying the circuit configuration and decreasing the cost.
Furthermore, the travel assisting hydraulic circuit for the hydraulic drive type working vehicle may include
a vehicle speed detector for detecting the rotational frequency of the drive wheel as a vehicle speed,
a hydraulic detector for detecting the circuit pressure of the assisting motor,
an engine speed detector for detecting an engine speed of the engine, and
a controller, and
the controller may obtain target circuit pressure relative to the detected engine speed of the engine when the detected vehicle speed is not lower than a predetermined assistance starting vehicle speed, and control an assisting quantity of the assisting motor so that the detected circuit pressure is equal to the target circuit pressure.
According to the above configuration, as an operating condition of the assisting pump and the assisting motor, a vehicle speed at which assistance is started, the pressure of the travel assisting hydraulic circuit, the engine speed, and the like are previously set, thereby the operation of the travel assisting hydraulic circuit can be automatically controlled in response to the traveling condition of the vehicle, Thus it becomes possible to travel at a speed much higher than when traveling only with the traveling HST circuit, during traveling at high speed without complicated operations by an operator.